It is often desirable to know the location of a terminal such as a cellular phone. For example, a location services (LCS) client may desire to know the location of a terminal in case of an emergency services call, or to provide location based services such as navigation assistance or direction finding. The terms “location” and “position” are synonymous and are used interchangeably herein.
In Observed Time Difference of Arrival (OTDOA) based positioning, the mobile station may measure time differences in received signals from a plurality of base stations. Because positions of the base stations can be known, the observed time differences may be used to calculate the location of the terminal. To further help location determination, Positioning Reference Signals (PRS) are often provided by a base station (BS) in order to improve OTDOA positioning performance. The measured time difference of arrival of the PRS from a reference cell (e.g. the serving cell) and one or more neighboring cells is known as the Reference Signal Time Difference (RSTD). Using the RSTD measurements for two or more neighbor cells, the absolute or relative transmission timing of each cell, and the known position(s) of BS physical transmitting antennas for the reference and neighboring cells, the position of the User Equipment (UE) or Mobile Station (MS) may be calculated. Using (i) the RSTD measurements, (ii) the absolute or relative transmission timing of each neighboring cell, and (iii) the known position(s) of BS physical transmitting antennas for the reference and neighboring cells, the UE's position may be determined.
A UE may encounter one or more LTE Inter-Frequency Handover (IFH) or Out Of Service (OOS) events during an OTDOA measurement session. OOS events lead to a loss of service. IFH events occur when one or more neighbor cells operate on a different carrier frequency than the serving cell. Upon encountering an IFH or OOS event, UEs may obtain or report: (a) RSTD measurements prior to the event; or (b) RSTD measurements subsequent to the event after resumption of LTE service, and prior to another event or expiry of the OTDOA session; or (c) no RSTD measurements, in situations where an inadequate number of measurements were obtained during periods of LTE service.
The exemplary situations outlined above may result in one of: (a) stale RSTD measurements, or (b) fewer RSTD measurements, or (c) no RSTD measurements, thereby detrimentally impacting UE position determination. For example, intermittent loss of LTE service during an emergency (e.g. 911) call may severely impact position determination thereby affecting delivery of emergency or other services.